Enhanced PAgP

Enhanced PAgP
With the introduction of Cisco Virtual Switching System in the first software release, an enhancement to the PAgP protocol (Enhanced PAgP or PAgP+) has been implemented to assist in the dual-active detection.

The result of this detection is that the standby virtual switch (switch 2) always transitions to become an active virtual switch and the active virtual switch (switch 1) always enters into recovery mode.

Upon the detection of VSL going down on switch 2, the switch will immediately transmit a PAgP message on all port channels enabled for Enhanced PAgP dual-active detection, with a Type-Length-Value (TLV) containing its own Active ID = 2. When the access switch receives this PAgP message on any member of the port channel, it detects that it has received a new active ID value and considers such a change as an indication that it should consider switch 2 to be the new active virtual switch. In turn, the access switch modifies its local active ID value to Active ID = 2 and immediately sends a message to both virtual switches on all members of the port channel with the new Active ID = 2 to indicate that it now considers switch 2 to be the active virtual switch.

Form this point onward, the access switch sends TLVs containing Active ID = 2 to the virtual switches in all its regularly scheduled PAgP messages.

Use the following commands to configure the Cisco Virtual Switching System to take advantage of dual-active detection using Enhanced PAgP:

vss#conf t
Enter configuration commands, one per line. End with CNTL/Z.
vss(config)#switch virtual domain 10
vss(config-vs-domain)#dual-active detection pagp
vss(config-vs-domain)#dual-active trust channel-group 20
vss(config-vs-domain)#

To verify the configuration and ensure that Enhanced PAgP is compatible with its neighbors, issue the following command:

vss#sh switch virtual dual-active pagp
PAgP dual-active detection enabled: Yes
PAgP dual-active version: 1.1
Channel group 10 dual-active detect capability w/nbrs
Dual-Active trusted group: No
Dual-Active Partner Partner Partner
Port Detect Capable Name Port Version
Gi1/8/1 No SAL0802SHG 5/2 N/A
Gi2/8/1 No SAL0802SHG 5/1 N/A
Channel group 20 dual-active detect capability w/nbrs
Dual-Active trusted group: Yes
Dual-Active Partner Partner Partner
Port Detect Capable Name Port Version
Te1/1/1 Yes vs-access-2 Te5/1 1.1
Te2/1/1 Yes vs-access-2 Te5/2 1.1

Action upon Dual-Active Detection

Upon detecting the dual-active condition, the original active chassis enters into recovery mode and brings down all of its interfaces except the VSL and nominated management interfaces, effectively removing the device from the network.

Recovery from Dual-Active Scenario
You are notified of the situation through the CLI, syslog messages, etc., and it is your responsibility to restore the original active virtual switch as part of the Cisco Virtual Switching System. You can restore it by reconnecting or restoring the VSL.

If a VSL flap occurs, the system recovers automatically. Upon a link-up event from any of the VSL links, the previous active supervisor engine that is now in recovery mode reloads itself, allowing it to initialize as the hot-standby supervisor engine. If the peer chassis is not detected because the VSL is down again, the dual-active detection mechanism determines whether or not the peer chassis is active. If the peer chassis is detected, this event is treated as another VSL failure event and the chassis once again enters into recovery mode.